Mr Knupfer, who was involved in the 1980s in the searches for the victims of the Moors murderers, says at least six and up to 12 people would have participated in each abduction, killing and secret burial of the disappeared. “Just to think about,” he says. “There must have been between a dozen and half-a-dozen people involved. There would be a driver, guards, people who dug the graves, often there would be pilot [scout] cars as well . . . ”

How could there not be people out there who could end the torment of the families of the victims, he wonders. Ten bodies have been recovered but seven remain missing. Of the seven, the commission has no leads on Joe Lynskey, a man in his 40s who disappeared from west Belfast in 1972, nor on Capt Robert Nairac, shot dead just south of the Border in 1977. Both were killed by the IRA.Former monkMr Lynskey was one of the first of the people know as the Disappeared to be killed but was one of the last to be known about. It was only three years ago that the IRA admitted he was one of these victims.

A former Cistercian monk, he was killed because, the IRA decided, he was seriously compromised due to an alleged relationship with the wife of an IRA prisoner. While the IRA admitted the killing in 2010, the commission still has not been able to persuade former members to provide details of where he is buried. Mr Knupfer was emphatic that the body of Capt Nairac – who was killed by the IRA in Ravensdale, Co Louth – was not destroyed, as has been reported and rumoured.

“We are absolutely confident that this story of his remains being put through a meat processing factory are just pure myth. We are happy that wasn’t the case,” he says. “We are confident he is buried south of the Border and that his remains are recoverable.”

South Armagh IRA members are notorious for their reluctance to yield up their secrets. Nonetheless, Mr Knupfer appealed to their humanity: “This is not about Robert Nairac, soldier, it is too late to help him; it is about his family who are grieving just like any other family would do.”

“A location was pointed out to us. We searched it and not only were we able to say Seamus Ruddy wasn’t there but that the ground had never been disturbed. The assumption we draw from that is it was the wrong place,” says Mr Knupfer.‘First-hand knowledge’The commission is convinced that Columba McVeigh, who went missing in 1975, is buried in Bragan bog in Co Monaghan, but despite several digs since 1999 his body has not been found. Mr Knupfer wants more specific information. “We believe there are other people out there who have first-hand knowledge, and we appeal for them to come forward,” he says.

The commission also believes Kevin McKee and Seamus Wright, who went missing from west Belfast in 1972, are buried in the bog at Coghalstown in Co Meath. But, again, finding more specific information is the problem. It has “high-grade information” that Brendan Megraw, who disappeared from Twinbrook in west Belfast in 1978, was buried in Oristown, Co Meath. There is a concern, however, that due to previous bulldozing his secret grave could have been destroyed, according to Mr Knupfer.

“We know in some cases there are people alive who we have not managed to speak to us yet; for whatever reason they won’t engage with the commission. It would be helpful to speak to people with primary knowledge,” he adds. Anyone with any information is guaranteed absolute immunity and anonymity, Mr Knupfer stresses.